Conventionally, a closed loop control method, such as speed feed back control or position feed back control, is well known as one of control methods of DC and AC motors. Generally, the above mentioned closed loop control is conducted by using an encoder or the like, in order to detect rotational amount of a motor, or driving amount of the driving target. Based on information from the encoder, respective feed back controls mentioned above are conducted.
However, when these feed back controls with an encoder are applied to a case wherein a driving target is driven at minimal velocity (that is, in case a motor is rotated at minimal velocity), it is possible to cause stoppage of the driving target. If the velocity of the driving target is beyond a target value in the time of calculation, a feed back process, wherein a control device makes control output small so that motion of the driving target is decelerated, is conducted. But when the driving target is driven at minimal velocity, the motion of the driving target is controlled with very small amount of control, and the driving target completely stops in an area wherein the feed back control is conducted. Thus, once the driving target stops, a change is not caused in signals from the encoder, and there is no operation generated to increase the control output in order to accelerate the velocity. In other words, controllability cannot be maintained unless the target velocity is set to be faster than the specified velocity. Therefore, it is difficult to apply the feed back control to controlling a driving target at minimal velocity.
A case wherein a carriage in an inkjet printer loading a recording head is moved to an initial position and stopped at the time of recording initiation, or a case wherein the carriage is returned to a home position to cap the recording head (nozzles) after recording is over, these cases can be specific examples of driving a driving target at minimal velocity. Among these two cases, the following describes the case of capping after recording, based on FIG. 70.
FIG. 70 is an explanatory view showing a schematic structure of a recording mechanism of an inkjet printer.
As shown in FIG. 70, a recording mechanism 100 of the inkjet printer comprises a guide shaft 101; a carriage 102 capable of reciprocating along the guide shaft 101; a recording head 103 loaded on the carriage 102; a transfer belt 104 which transmits driving force from a motor 110 to the carriage 102; and an encoder 105 which detects transfer amount/position of the carriage 102.
The motor (DC motor) 110 is rotated by an ASIC 111 outputting driving signals according to various commands from a CPU 112, and drives the transfer belt 104 which is endless and disposed parallel to the guide shaft 101. This driving force is transmitted to the carriage 102, and the carriage 102 and the recording head 103 are reciprocated along the guide shaft 101. On the carriage 102, ink tanks of respective colors are loaded (not shown). Ink of respective colors reserved in the ink tanks is respectively jetted out to a recording paper a from nozzles 107 of the recording head 103.
The encoder 105 is a known linear encoder which outputs two types of pulse signals having different phases corresponding to movement of a transfer detection target (the carriage 102 in this case). Although the detail is not shown in the drawing, an encoder strip wherein plural slits are formed with specified intervals therebetween is disposed along the guide shaft 101. Two types of pulse signals generated corresponding to the movement of the carriage 102 are inputted to the ASIC 111, and these signals are used as position/velocity information for controlling the motor 110.
This recording mechanism 100 furthermore comprises a capping device 106 which is to inhibit the ink from getting dried by covering all the nozzles 107 of the recording head 103. This capping device 106 is disposed in an stand-by area which is outside of a recording area where recording (printing) on the recording paper a is conducted. The capping device 106 comprises a slope 123 which is formed so as to become upward toward the outside (the right side in the drawing); a cap 121 movable on the slope 123; and a spring 122 which pulls the cap 121 toward one side of the slope 123 declining downward.
The carriage 102 comprises a hook (not shown). When the carriage 102 moves in the stand-by area toward direction A shown with an arrow, firstly the hook is hooked on the cap 121. Subsequently when the carriage 102 furthermore moves toward the right end of the stand-by area, corresponding to this movement, the cap 121 is pulled to the right side along the slope 123, and the nozzles 107 are gradually covered by the cap 121. When the carriage 102 reaches the home position on the right end, the cap 121 completely covers the nozzles 107.
Capping, wherein the cap 121 covers the nozzles 107 as described above, is generally conducted when recording operation on the recording paper a is completed and the recording head 103 is going to be returned to the home position. If the nozzles 107 are not covered completely by the cap 121, the ink gets dried. In order to conduct a steady capping, the carriage 102 is stopped once when entering the stand-by area from the recording area after recording operation, and moved to the home position at minimal velocity.
Meanwhile, when recording operation is initiated, the carriage 102 is moved from the home position to a gap adjustment area so as to abut on the left end thereof, subsequently the carriage 102 is moved (returned) for predetermined distance toward the direction A and stopped. Thereby the carriage 102 is set on the initial position for recording initiation. When recording operation is initiated, in the gap adjustment area, the carriage 102 is moved at minimal velocity in order to control the force of the carriage 102 to abut on the left side of the gap adjustment area, and in order to set the carriage 102 on the initial position for recording initiation.
It is to be noted that the gap adjustment area is an area wherein a gap adjustment device (not shown) can be operated. By operating this gap adjustment device, a gap between the nozzles 107 of the recording head 103 and the recording paper α can be adjusted.
As described above, if the feed back control is applied to a case wherein a motor drives a driving target at minimal velocity, such as driving the carriage 102 at minimal velocity in the above-described inkjet printer, various problems can be caused.
Consequently, in order to drive a DC motor, for example, at minimal velocity, there is a known control method. In this method, every time an edge of pulse signals from an encoder (to be referred to as an encoder edge) is detected, driving force given to the DC motor is increased from an initial value by a specified amount in a specified period. Specifically, in this control method, distribution of electric current to the DC motor is controlled by PWM control, and a PWM duty value (to be referred to as PWM value) is increased from the initial value by the specified amount in the specified period at the time of every encoder edge detection (e.g. disclosed in Unexamined Japanese Publication No. 2003-79189 (FIG. 3)).
An example of controlling a DC motor by using this control method is going to be described based on FIGS. 71A and 71B. FIG. 71A shows an example of control from a step of decelerating a driving target driven by a DC motor while the driving target is driven at minimal velocity, to a step of stopping the target after initiating braking. When this motor control is initiated, a PWM value is set to a preset PWM value for driving initiation (start_pmw1), and increased by the specified amount in the specified period.
The way of this increase is shown in FIG. 72. Every time an encoder edge is detected, that is, every time encoder edge detection signals are outputted, a PWM value (pwm_out) is reset to start_pwm1, and increased by a specified incremental value (accel_param) in specified period Tp. To be more specific, the specified period Tp, in which a PWM value is renewed. i.e. PWM value renewal period Tp is shown as Tp=pwm_period‡(pwm reload_count+1). Pwm period indicates the period of PWM signals, and pwm_reload_count indicates timing for constant addition, which shows the number of PWM pulses outputted between reset of a PWM value and a next renewal.
Referring to FIG. 71A again, the PWM value is increased from start_pwm1 by the specified amount (accel_param) in the specified period Tp, as described above. In this example, the maximum value of the PWM value (pwm_limit) is set, and a PWM value does not go beyond the maximum. Hence, when a PWM value reaches pwm_limit, the PWM value is maintained to be pwm_limit. When an encoder edge is detected, the PWM value is reset to start_pwm1, and once again increased by the specified amount (accel_param) in the specified period Tp.
If a position of the driving target at the time of encoder edge detection is at a deceleration starting position (decel_pos), the PWM value is not set to start_pwm1, but to start_pwm2, which is a PWM value for deceleration initiation. Thereafter, in the same manner as shown in FIG. 72 (nevertheless, the PWM value is not increased, but decreased in the specified period), the PWM value is decreased. When a predetermined condition for braking initiation is met, braking is initiated.
When braking is initiated, the PWM value is set to start_pwm3, which is a PWM value for braking initiation. Subsequently, the PWM value is decreased by the specified amount in the specified period. When the PWM value reaches stop_pwm, which is a PWM output value during braking, the PWM value is maintained.
In this status, if the PWM value is a plus (+) value, such as during normal drive or deceleration, electric current distributed to the DC motor is switched on/off according to duty ratio corresponding to the PWM value. On the other hand, if the PWM value is a minus (−) value, a short circuit is formed, and blocking and conduction of the short circuit is conducted according to the duty ratio corresponding to the PWM value. That is, as the PWM value becomes smaller in the minus area, the ratio of conductivity in the DC motor short circuit becomes larger, and braking force increases. When the PWM value becomes stop_pwm, the short circuit becomes constantly shorted and the braking force becomes the maximum.
According to the above-described DC motor control method, when the driving velocity of the driving target increases, the intervals in encoder edge detection becomes short, and a PWM value is reset again to the initial value start_pwm1 while the driving force (PWM value) is not increased sufficiently. In other words, in this control method, when velocity increases, driving force of a motor consequently decreases. Hence, it is an open control, on the whole, nevertheless, stable control is achieved to certain extent.
Therefore, for the motor 110 of the recording mechanism 100 of the inkjet printer described in FIG. 70, when the carriage 102 is driven/controlled at minimal velocity for capping operation, this control method, wherein a PWM value is increased from an initial value by a specified amount in a specified period at every encoder edge detection as described above, is adopted.